1. Field of the Invention
This invention relates to a switching constant-current power device for supplying stabilized current to a repeatedly interrupted load.
2. Description of the Prior Art
Conventionally, switching-system power devices are often used in supplying a stable voltage to a load. However, a switching-system power device having the circuit constitution shown in FIG. 1 can also be used in supplying an approximately constant current to a load, by supplying a feedback signal in correspondence with an output current to a control circuit.
In FIG. 1, reference numeral 1 represents an input terminal, which electrical power is supplied to from an outside battery or the like, and reference numerals 2a and 2b represent output terminals for stably supplying a predetermined current to a load 6, provided between them. A power converter 3 is connected between the input terminal 1 and the output terminal 2a, and forms a boosting chopper converter comprising a choke coil L1, a switching transistor Q1, a rectifying diode D1, and a smoothing capacitor C1.
A current detector 5 is connected between the other output terminal 2b and ground, which functions as a reference potential point for the circuit; the current detector 5 detects current flowing to the load 6 (hereinafter “load current”), and generates a feedback signal in correspondence therewith. A controller 4 is connected between the power converter 3 and the current detector 5, receiving the feedback signal from the current detector 5 and driving the power converter 3 in accordance with the level of the feedback signal. (It is assumed here that the controller 4 comprises a conventional IC using a separately excited PWM control system.)
The power converter 3, the controller 4, and the current detector 5, together constitute a switching constant-current power device. The element CO connected between the input terminal 1 and ground is a capacitor for input filter.
The operation of the switching constant-current power device of FIG. 1 will be explained briefly. The switching transistor Q1 in the power converter 3 switches on and off in compliance with a signal supplied from the controller 4 (the on-off frequency being approximately several hundred kHz). In compliance with the on-off operation of the switching transistor Q1, current is fed from the choke coil L1 via the rectifying diode D1 to the smoothing capacitor C1. As a result, the smoothing capacitor C1 is charged to a higher voltage than the input voltage supplied to the input terminal 1, and a current corresponding to the voltage between the terminal of the smoothing capacitor C1 flows between the load 6 and the current detector 5. The current detector 5 creates a feedback signal in correspondence with the load current, and feeds it back to the controller 4.
The level of the feedback signal supplied from the current detector 5 to the controller 4 corresponds not to the output voltage, as in a normal switching power device, but to the output current (=load current). Therefore, in compliance with its own internal control logic, the controller 4 creates a high-frequency (several hundred kHz) on-duty pulse signal in correspondence with the feedback signal (=load current), and supplies this signal to the switching transistor Q1. The switching transistor Q1 switches on and off at an on-duty corresponding to the size of the load current. For example, when the load current is lower than the stabilizing target value, the voltage between the terminals of the smoothing capacitor C1 is boosted and induced so as to increase the load current. As a result of such operations, the load current of the device shown in FIG. 1 becomes stable.
Recently, however, display devices and lighting devices of various sizes and types are being fitted in electronic equipment, and many of these devices use a light-emitting diode (hereinafter “LED”) as their light source. When using an LED as a light source, a stable current supply is required in order to ensure that the amount of light, brightness, and the like, are constant. Accordingly, in recent electronic equipment, a switching constant-current power device such as that shown in FIG. 1 is provided in accompaniment with the display devices and lighting devices, and supplies stabilized current to the LED, as disclosed in Patent Documents Nos. 1 to 3.                Patent Document No. 1: Japanese Laid-Open Patent Application No. 11-068161        Patent Document No. 2: Japanese Laid-Open Patent Application No. 2001-215913        Patent Document No. 3: Japanese Laid-Open Patent Application No. 2002-203988        
In some recent display devices and lighting devices that use an LED as a light source, the LED is repeatedly switched on and off at a speed that cannot be perceived by the human eye (specifically, several hundred Hz or more) to reduce energy consumption and adjust the light. Display devices and light devices of this type naturally have periods where current is flowing to the LED (hereinafter “current pass period”) and periods when it is not (hereinafter “current cut-off period”). When current is being supplied to the LED by a switching constant-current power device such as that shown in FIG. 1, the feedback signal supplied from the current detector 5 to the controller 4 is almost at zero level during the current cut-off period that is created by interrupting the load.
In response to this feedback signal, the controller 4 attempts to set the on-duty of the on-off operation of the switching transistor Q1 to its maximum during the current cut-off period, and attempts to set the on-duty in correspondence with the feedback signal in the next current pass period. When the on-duty is at its maximum during the current cut-off period, the voltage between the terminals of the smoothing capacitor C1 rises abruptly more than is necessary, and a load current exceeding the target value for stabilization flows for a comparatively long time during the next load current pass period, creating instability in the load current.
One method of countering this type of instability in the load current is, for example, to smooth the feedback signal by using a capacitor having a comparatively large capacity before supplying it to the controller 4. However, when a capacitor having a capacity large enough to keep the feedback signal at a significant size during the current cut-off period is provided, the feedback signal processed by the controller 4 holds the average value for a comparatively long time. Consequently, non-cyclical load interruption, or fluctuation in the load caused by factors other than interruption, makes it impossible to speedily return to the stabilization target value a load current that has deviated from that target value, and may result in load current instability caused by factors other than the current cut-off period.
Thus, with an interrupted load, the response speed of the control operation of the feedback loop running from the controller 4 via the switching transistor Q1, the smoothing capacitor C1, the load 6, and the current detector 5, to the controller 4, may become incapable of keeping up with changes in the load, making it impossible to stabilize the load current.